The present invention relates generally to diode laser array systems. More specifically, the present invention relates to high efficiency, high power direct diode laser systems.
In numerous applications such as laser tracking, laser guidance and laser imaging, it is desirable to produce a high power coherent laser output. Moreover, high power coherent laser systems find applications in such diverse fields as offensive and defensive weapon systems, e.g., non-visible light illuminators for special operation forces and protective laser grids, as well as material processing, e.g., welding, cutting, heat treating and ablating, and medicine, e.g., surgical and diagnostic aides.
In the earliest laser systems, single semiconductor lasers were utilized to provide a coherent source of laser output. These single semiconductor lasers were limited in the amount of power which they could provide due to their structural limitations and limited efficiency. More recently, arrays of semiconductor lasers have also been utilized in which adjacent emitters of the array of semiconductor lasers spaced upon the same substrate are coupled together. One such laser array system was disclosed in commonly assigned U.S. Pat. No. 5,212,707 to Heidel et al., which is incorporated herein by reference for all purposes.
FIG. 1 illustrates a one-dimensional semiconductor laser array 10 according to U.S. Pat. No. 5,212,707, which is mounted on a heatsink 12. The semiconductor laser array 10 has an associated lens assembly 22, 24 for collimating the laser array's output, which is positioned adjacent to the emitting facet of the semiconductor laser array 10. Lens assembly 22, 24 is attached to the ears 25 of the heatsink 12. The emitters 20 of the array 10 are supplied with power from an external power supply via wires 18, a standoff pad 16 and a power lead 14. In an exemplary case, the semiconductor laser array 10 shown in FIG. 1, includes ten individual emitters 20; and number of emitters 20 may be employed as determined by the requirements of the particular application.
Once the semiconductors laser array 10 has been fabricated, mounted and powered, the output of the semiconductor laser array's emitters 20 must be collimated in order to obtain the desired collimated output. The lens assembly, as shown in FIG. 1, which is designed to collimate the output of the semiconductor laser array 10, includes a first refractive lens 22, typically of a biconvex design, and a second binary optical element 24, which is essentially a diffractive lens. The refractive biconvex lens 22 collimates the fast axis of each emitter 20 while the binary optical element 24 serves to collimate the slow axis of each emitter 20 and correct all spherical aberrations including those introduced by the collimation performed by the refractive lens 22.
The binary optical element 24 includes a substrate on which a binary optical diffraction pattern 26 is etched. Generally, the materials of the refractive lens 22 and the binary optical element 24 have substantially equivalent refractive indices such that minimal refraction occurs at the interface between the refractive lens 22 and the binary optical element 24. The binary optical element 24 has a back surface 27 positioned adjacent to the front surface 28 of the refractive lens 24 and a front surface 28 on which the binary optic diffraction pattern 26 is etched. Since the binary optical diffraction pattern 26 is produced in accordance with typical binary optic technology, as well known to those of ordinary skill in the art (See U.S. Pat. No. 4,846,552.), further discussion of this technology will not be provided.
The binary optic diffraction pattern 26 is typically an eight phase level structure (although a two, four, or sixteen-phase level structure could also be utilized) which corrects for optical path differences inherent in the divergent output light of an emitter of a semiconductor laser array. Thus, the rays of light which exit the binary optic element 24 will have all travelled equal optical pathlengths, defined as a physical pathlength multiplied by the index of refraction of the material through which the light rays travelled which are equal or varied from that equal optical pathlength by only an integer multiple of the wavelength of the light being emitted. An eight level binary optic diffractive pattern 26 is shown schematically in FIG. 1.
A two-dimensional semiconductor laser array can be fabricated from a plurality of the one-dimensional semiconductor laser arrays 10 shown in FIG. 1. The one-dimensional semiconductor laser arrays 10 are stacked as shown in FIG. 2 within a heatsink which serves as a holding or clamping fixture 70. The clamping fixture 70 is designed such that the one-dimensional semiconductor laser arrays 10 may be stacked on top of one another so that the outputs of each one-dimensional semiconductor laser array are substantially parallel to the outputs of the other semiconductor laser arrays.
Once the one-dimensional semiconductor laser arrays 10 have been mounted within the clamping fixture 70, the collimating lenses are aligned and attached. The fabrication of the collimating lenses is done in a manner identical to that previously discussed such that the refractive lens 22 is cemented to the binary optical element 24 which has been designed to collimate the laser output of each emitter 20. The alignment and attachment of the collimating lenses is accomplished in a sequential fashion for optimum efficiency. The collimating lenses 80a associated with the first one-dimensional semiconductor laser array 10a are positioned as previously described such that the optical axes of each emitter 20 of the semiconductor laser array 10 are substantially aligned with the center of the collimating lens assembly 80a.
The second collimating lens assembly 80b is then placed in front of a second one-dimensional semiconductor laser array 10b and is held in position by means of a vacuum chuck 76 connected by a vacuum line to a vacuum source, as shown in FIG. 3. The two-dimensional semiconductor laser array 10 is then supplied power such that the emitters 20 produce a light output. A transform lens 72 is positioned within the path of the light emitted from the first and second one-dimensional semiconductor laser arrays. The transform lens 72 may be a plano-convex or a biconvex lens, as shown in FIG. 3, such that a simulated far field will appear at the focal plane of the transform lens 72 when the input light to the transform lens 72 is collimated. To determine the simulated far field, when all beams of light overlap at the focal plane of the transform lens 72, a line scan detector 74 is positioned at the focal plane. The output of the line scan detector is monitored to determine if proper collimation has been achieved. The position of the second collimating lens assembly 80b is varied until proper collimation is observed at the focal plane of the transform lens. Once proper collimation is observed, the position of the second collimating lens assembly 80b is preserved by fixing the lens assembly in position in the ears 25 of the clamping fixture 70. An identical alignment procedure is done for each lens assembly and its corresponding one-dimensional semiconductor laser array 10, until the lens assembly for each semiconductor laser array 10 has been properly aligned such that the light is collimated and focused at the simulated far field.
The two-dimensional laser array when properly supplied with power produces a single collimated spot of laser output in the far field. By utilizing a plurality of one-dimensional semiconductor laser arrays 10 whose outputs may be combined, the output power of the two-dimensional semiconductor laser array may be quite high. For example, 25 watts of continuous wave laser energy was produced by a two-dimensional semiconductor laser array consisting of twelve one-dimensional semiconductor laser arrays with each one-dimensional semiconductor laser array having twenty one emitters. Additionally, the overall efficiency of the laser array from electrical input to power in the central lobe was approximately 26%.
U.S. Pat. No. 5,299,222 discloses an alternative approach to producing a high power laser diode system that collects and concentrates laser output from a stack of diode laser bars in a form that is useful and flexible for pumping a laser, e.g., a solid state laser. As shown schematically in FIG. 4, the light beam output of stacked diode laser bars is coupled into a plurality of optical fibers. The output light beams from the fibers may be used to pump a laser resonator. The fibers can be grouped at various end points of a solid-state laser cavity for efficient end-pumping. In FIG. 4, a light beam 11 is emitted by a plurality of diode laser bars in a diode laser bar stack 13, and light from a selected group of the bars is collected by one of a plurality of cylindrical lenses 15 positioned adjacent to but spaced apart from each diode bar in the stack 13. Each diode laser bar may have an aspect ratio (length-to-width) as high as 10.000:1, or even higher, and the cylindrical lenses 15 are interposed to reduce the beam divergence angle in a first direction, relative to the beam divergence angle in a second, perpendicular direction, so that the resulting beam divergence angle in each of the two directions is roughly the same.
Two or more turning mirrors 17A, 17B, 17C and 17D separate mutually exclusive portions of the light beam 11 into non-overlapping light beam components 19A, 19B, 19C and 19D, respectively, and at least one pump light beam component, such as 19E, is optionally defined by a portion of the light beam 11 that does not encounter a turning mirror. Each light beam component 19A, 19B, 19C, 19D and 19E is then focussed by suitable focusing optics 21A, 21B, 21C, 21D and 21E, respectively, into a corresponding multimode optical fiber 23A, 23B, 23C, 23D and 23E, respectively, with the diameters of the fibers being chosen to fully capture the optical beam intended for that fiber. Preferably, the sine of the convergence angle as a light beam arrives at a light-receiving end of a fiber is less than the numerical aperture NA of that fiber. In one embodiment, each optical fiber has a diameter of about 500 μm, but this fiber diameter may be as large as a few mm. Each of the focusing optics 21j (j=A, B, C, D or E) may be a lens with a short focal length, such as f=6.35 mm, and is intended to cause the resulting beam to converge to an entrance diameter, measured at the entrance of the corresponding fiber 23j, that is about 25 percent of the diameter of the portion of the pump light beam 11 that arrives at the focusing optics 21j.
The numerical aperture NA of the multimode fiber 23j lies in the range 0.15-0.3 but may be as high as 0.6. Each optical fiber 23j delivers the component pump light beam propagating therein to a selected position and with a selected angular orientation relative to the laser cavity to be pumped by this collection of component pump light beams. Each optical fiber 23j is provided with an anti-reflective coating at the diode laser wavelength P, and the coating is either applied directly to the fiber end or to a separate glass window that is bonded to the light-receiving end of that fiber. The core material of the fiber 23j may be glass, and the cladding material of the fiber may be glass or plastic, with a smaller refractive index than the core refractive index, which determines by the numerical aperture of the fiber in a manner well known in the art.
It will be appreciated that expansion of the systems discussed immediately above would require both a large amount of real estate and complex optic assemblies to couple the outputs of a plurality of the disclosed output modules to a single spot. For example, the presence of lens 72 in FIG. 3 suggests the need for a focusing lens associated with each module; FIG. 4 suggests that a plurality of lenses 21 are needed to efficiently couple the output of a single diode laser array. It would be desirable for a plurality of semiconductor laser arrays to produce a single spot of high intensity laser output using a simple and robust optical subsystem. Furthermore, it would be desirable for a plurality of semiconductor laser arrays to be mounted evenly and the outputs thereof collimated in such a manner as to fill the available aperture to thereby provide a substantially constant intensity across the single spot of laser output produced.